18% grey is used in the grey card as a base color. Both grey and white surfaces of the grey card have their own function. Whatever you used in film or digital photography, an 18% grey card can control expose value and balance the color so that it helps to revert the color truly and perfectly by measuring the exposure value, white/grey balance value and color temperature.
A grey card had been invented based on the optical principle and the characteristic of photosensitive film. The camera exposure value is in direct ratio with the logarithm of characteristic curve. Characteristic curve shows two important features of photosensitive material (they are: contrast factor and exposure latitude), and it is in coordinate with the sense of human sight in reflection to the colors in ray. When color changes as geometric progression, its density will change as arithmetic progression. If we try to get a series of density values like 1:2:3:4:5……, the exposure value will be increased like 1:2:4:8:16…., always in multiple.
If we set the reflection ratio to 100% (as maximum) and decrease progressively as common ratio of 2, we’ll get the density value of full-level diaphragm: 100, 50, 25, 12.5, 6.25, 3.125…… if the reflection ratio decrease progressively as square root of 2, we’ll get the density value of half-level diaphragm: 100, 70.71, 50.35, 36.25, 17.68, 12.5, 8.84, 6.25, 4.42, 3.125…… Actually those values are just different grey values in different grades. We take it that the standard exposure value is 17.68, then those values beside it will be its exposure latitude.
Look at those density values above, from number 100 to 3.125, the ratio is 32:1. The exposure latitude in this ratio is exactly included in the photosensitive range among in color negative, black/white negative, color reversal film and CCD. We’ll see that 17.68 is just right in the middle of those series of numbers. Thus, it will be excellent and precise to take 17.68 (approximation of 18%) as a base value, which controls the exposure value, photographic density and color balance.
In manufacturing of photosensitive products, 18% grey had become a standard in theory because of its special characteristic. 18% grey as a foundation used in the photographic sensitivity of the film, the exposure value of exposure photometer, CCD and CMOS sensors.
Specification:
Package Contains:
.webp "3 in 1 Digital White Black Grey Balance Cards")
